7.1.1　Technical Difficulties

In the process of slurry pressurized shield advancing in the middle channel section of the river，due to the shallow overburden soil and poor soil quality，the stability of the shield excavation face is difficult to control，and it is easy to fall and collapse.In order to prevent roof fall and collapse，the support pressure amplitude must be accurately controlled.If the amplitude is not properly controlled，it is easy to cause a small range of variation between the minimum limit and the maximum water pressure，making the support pressure of the excavation face difficult to control.At the same time，because of the high water pressure and large permeability in the channel section，it is difficult to set up the upper pressure due to the influence of tide and other factors in the river，which will cause the difficulty of mud pressure setting.Slurry pressure setting is too large，prone to pressure mud breakdown of shallow soil layer，to the formation of split penetration，mud to shield tail gap flow channeling and other mud loss risk.When the slurry pressure is set to be small，it is easy to occur the accident that the river water pours into the excavation face and causes the instability of the excavation face.When the shield passes through the sandy silt layer of shallow overburden，the stratum passed by the tunnel is mostly saturated muddy clay，silty clay and sandy silt，and the stratum is mostly soft plastic.In this stratum，there may be a slight liquefied soil layer，which is prone to flow sand and water gushing under certain hydrodynamic conditions，and seriously will cause the collapse of the soil ahead of the excavation.In addition，the lining segment is affected by the buoyancy of surrounding groundwater and shield tail grouting slurry.When the upper soil pressure and the weight of the segment cannot resist the buoyancy of the segment，the segment will float and rupture.When the thrust section in the middle of the Yangtze River is advancing，the force of the shield is unbalanced，and the axis is difficult to control.The asynchronous steering between the shield and the segment in the curve section will make the shield gap difficult to control，and it is prone to uneven phenomenon of segment gap，resulting in water balance，slurry leakage or segment shell.

In summary，the main technical difficulties of shield tunneling through the middle channel section of the Yangtze River are：①The thickness of the overburden is small，the water pressure is large，the stratum mutation，the stability control of the excavation face is difficult，and it is easy to fall and collapse.②It is difficult to set the mud pressure.If the mud pressure is too large，the soil layer will be split.If the mud pressure is too small，the river water will flow into the excavation surface and cause instability.③The soft plastic soil layer is easy to be liquefied under hydrodynamic conditions，resulting in sand gushing and water gushing.At the same time，when the shield tail sealing effect is poor or the grouting pipe is cleaned，water gushing and sand gushing may also occur.④It is inevitable to cause over-excavation during shield tunneling，and it is easy to cause water balance at shield tail，slurry leakage or segment shell sticking due to uneven segment gap.⑤When shield tunneling，the upper and lower forces are unbalanced，and the axis is difficult to control，which is prone to tunnel floating and segment damage.